1. Field of the Invention
The present invention generally relates to variable speed, constant frequency aircraft power generating systems, and more particularly to an improved fault detection circuit for such systems.
2. Description of the Prior Art
In an aircraft power generating system employing a variable speed, constant frequency control, a three phase power generator is driven by the aircraft engine turbine. A power transmission line couples the generator output to a full wave rectifier whose DC output is coupled to an invertor. In order to detect a fault between the generator and the rectifier/inverter, fault detection systems have been proposed which use a current transformer in each leg of the generator to sense the input current to the power line and a second set of current transformers to sense the input current to the rectifier. The phase currents at each end of the power transmission line are compared and an unbalance current indicates a fault. While generally satisfactory in operation, such prior art fault detection circuits are relatively costly, and more importantly, the use of current transformers adds weight to the aircraft power generating system.
Power semiconductor devices having a main current section and a current sensing section that follows the current in the main section have recently been developed. Insulated gate bipolar transistors and diodes with such current sensing have been proposed to provide overcurrent threshold detection and as current controlled inverter switches for an AC polyphase motor drive; for example, U.S. Pat. No. 4,777,579. However, they have not been used for fault detection in variable speed, constant frequency aircraft power generating systems.